As petroleum reserves become depleted and more expensive, the need for alternative, and preferably sustainable, energy sources increases. For some years, ethanol has been considered and has been used as an option for partial or complete replacement of petroleum-based fuels for different applications. Ethanol-powered automobiles are a reality. Ethanol has advantages over the use of conventional gasoline as a renewable fuel source.
Ethanol is a major chemical product which has been produced by humans for millennia from natural sources. Currently ethanol is produced on a large scale from natural sources by a fermentation process in which sugar is converted to ethanol and carbon dioxide by yeast. Many feedstocks can be used to provide the sugar for fermenting. Current natural sources include corn, milo, wheat, barley, millet, straw, sorghum, sugar cane, sugar beets, molasses, whey, and potatoes. In fact, any starch or cellulosic material, which includes nearly all plants, can be used as a source of sugar for use in producing ethanol, as starch or cellulose can be a precursor to sugar.
An important concern with conventional fermentation systems is the difficulty in preventing microbial contamination, especially bacterial infection. Unfortunately, the optimum atmosphere for fermentation is also extremely conducive to bacterial growth. Bacteria can convert sugar (glucose) to organic acids, such as acetic acid and lactic acid, rather than ethanol. Furthermore, bacteria grow rapidly in the nutrient rich environment of a fermentation system, and may consume sugar (glucose) faster than does yeast. Furthermore, organic acids produced by the bacteria inhibit performance and growth of yeast. Thus, bacterial infection results in decreased yield of ethanol, and the fermentation process becomes less economical.
Current industry strategies to combat bacterial infection in fermentation systems include monitoring for the presence of organic acids (e.g., acetic acid and lactic acid) followed by remedial treatment. That is, once acids are detected, antibiotics or biocides may be added to control bacterial growth. However, bacterial growth and infection is a recurring problem. Any feed to a fermentation system, such as water, mash, enzymes and yeast as well as the fermenting vessel itself (if not disinfected between batches) can be a source of bacteria. Therefore, frequent monitoring is necessary and repeated introductions of antibiotics may be required.
Use of antibiotics to reduce bacterial growth in a fermentation system has become disfavored. Certain antibiotics remain and accumulate in solid products of fermentation, if they are not deactivated upon reaction with target bacteria. Solid products include distillers dried grain solids (DDGS) and distillers wet grain solids (DWGS). DDGS and DWGS are valuable byproducts of fermentation and are used in animal feeds. In many countries the amount of antibiotics in animal feed is under or being considered for regulatory control.
Generally, biocides perform poorly in fermentation systems, because they are non-specific and may also attack yeast. Stabilized chlorine dioxide (SCD) is a biocide that has been used in fermentation systems to treat bacterial infection. While yeast appears to be unaffected, this treatment is remedial, that is, only after the system has become infected. Repeated additions may also be required as indicated above.
U.S. Pat. No. 4,929,365 describes a remedial treatment process to remove microorganisms and biofilm produced by such microorganisms which then inhabit the biofilm, from a submerged substrate in an aqueous environment. The process uses stabilized chlorine dioxide (SCD), which is introduced to the substrate and allowed to penetrate through the protective biofilm and into the microorganism layer. A nutrient source is needed to create an acidic environment within the biofilm. This acidic environment activates the SCD, which in turn kills and destroys the microorganism and biofilm from the submerged substrate.
An alternative to remedial treatment is to prevent growth of bacteria. Addition of antibiotics in amounts to prevent growth of bacteria has been used. However, the issue of antibiotics accumulating in fermentation solids remains. Development of bacterial resistance is also a well known consequence of antibiotic use.
Thus, there is a need to prevent bacterial growth in a fermentation process, while minimizing or eliminating the use of antibiotics. The present invention meets this need.